Moldable silicon nitride green-body composite and reduced density silicon nitride ceramic process

ABSTRACT

A moldable green-body composite includes milling silicon nitride powder with a solvent and adding a surface modifier to the milled slurry to modify a surface of the silicon nitride particles. A polysiloxane in a solvent and a binder are also added to create a green body slurry. The solvents may be polar or non-polar solvents. A sintering aid, such as yttria-alumina, may be added to the slurry as well. A reduced density silicon nitride ceramic is made from the moldable green-body composite by molding the moldable green-body composite in a mold and curing at a curing temperature to convert the moldable green-body composite to a converted composite. The converted composite can then be sintered to form a reduced density silicon nitride ceramic that has a smooth surface finish and requires no post machining or polishing. The reduced density silicon nitride ceramic may also have very good dielectric properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. provisionalpatent application No. 62/533,771, filed on Jul. 18, 2017; the entiretyof which is hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION Background

Silicon nitride is a ceramic with, among other things, interestingmicrostructural characteristics, desirable electrical properties forradio frequency (RF) applications (i.e., radomes, windows and conformalantenna parts) and high temperature capability. While silicon nitride ismost commonly used in the structural ceramic market as bearings andother structural parts, it has long been considered a desirable materialfor elevated temperature RF applications. This is due to its strength athigh temperature and potential for very good dielectric properties.

Today there are two common routes to forming bulk silicon nitride parts.The first is by sintering silicon nitride powder, with or withoutapplied pressure (by far the majority of structural parts are made bypressure-applied sintering, either hot pressing or hot isostaticpressing) and the other is by the reaction bonding method.

In the sintering process, commonly known as sintered silicon nitride orsometimes hot pressed silicon nitride, a powder compact is formed andthen sintered at high temperature. Usually a large amount of sinteringaids is used to aid in achieving high densities. Because of this, thedielectric properties are not as desirable as could be.

In the reaction bonding process, known as reaction bonded siliconnitride (RBSN), a powder compact of elemental silicon is reacted withnitrogen gas to form silicon nitride. While this process yields partswith good dielectric properties, the process is difficult to control andexpensive. Sintered silicon nitride produces parts with relatively highdensity whereas RBSN produces parts with low density. As a result, thestrength of a low density RBSN part is lower than that of sinteredsilicon nitride.

In both processes, silicon nitride parts are formed oversize, sinteredor nitrided, and then post sinter machined to meet final part geometryand tolerances. This is due to the fact that tolerances, typically,cannot be held in these processes and, in addition, an undesirablereaction layer is formed on the surface of the parts which needs to beremoved post sinter.

Ceramics, including silicon nitride, are difficult to machine as theyare very hard and as a result material removal rates are very slow. Inaddition, silicon nitride is prone to cracking and chipping duringmachining. In many situations, a sintered silicon nitride ceramic partrequires extensive post-sinter machining to meet design tolerances andsurface roughness specifications. The cost of post surfacing thesintered molded part can easily exceed any of the prior manufacturingcost.

A moldable green-body composite with rheology that allows it to flowwell, typically requires an amount of binder, the component that enablesflow in the ceramic slurry, that is equal to 30 to 50 vol percent of theoverall composition. With clever use of particle size distribution, thisamount can be reduced to nearly 10 volume percent. The amount of effortto achieve this, however, is not feasible from a practical standpoint.In the end, with a traditional binder, it needs to be removed from thecomposite before final densification of the part. For parts with largecross-sections, even 10 vol. percent represents a difficult de-binding(burnout) challenge. In addition to the difficulties that a large amountof binder presents during processing, the large amount of binder cannegatively impact the strength and density of the sintered part and italso can act as a contaminate that degrades the dielectric properties.

During densification or sintering, ceramics, including silicon nitrideparts, shrink substantially during the sintering process making itdifficult to maintain important dimensional aspects that can be criticalfor an application. In particular, bolt or fastener locations or spacingtherebetween, and thread pitch and geometry of a threaded hole havecritical tolerances that are difficult to achieve with large andvariable shrinking characteristics.

SUMMARY OF THE INVENTION

The two most desirable attributes of this material/process is that partscan be made with minimal or no post-sinter machining and the electricalproperties for RF applications are superior to other silicon nitridematerials. These properties include: dielectric constant, loss and thedependence of these with temperature.

The invention is directed to a moldable green-body composite and areduced density ceramic composite. In an exemplary embodiment, themoldable green-body composite comprises silicon nitride that isconfigured to flow into molds having small aspect elements and uponsintering produces a reduced density ceramic that has a high strength todensity ratio. In addition, the sintered silicon nitride ceramic parthas minimal and predictable shrinkage and therefore allows molding ofparts with small aspect elements including threaded fastener holes.Furthermore, the silicon nitride ceramic parts produced from themoldable silicon nitride composite, described herein, has a smoothsurface finish eliminating or greatly reducing the need for postpolishing or grinding. The ceramic parts produced from the moldablegreen-body composites may also have a low dielectric constant, low lossand minimal variation of these with temperature, in part due to the lowdensity as well as the sintering aid chemistry involved.

An exemplary moldable green-body composite is made by ball milling aceramic, such as silicon nitride, down to a small average particle size,such as no more than about 20 μm, preferably no more than about 10 μm,more preferably no more than about 5 μm, more preferably no more thanabout 2 μm, and even more preferably no more than about 1 μm, and anyrange between and including the particle sizes provided. The siliconnitride particle size must be small enough to enable flow of themoldable green-body composite into the mold and small cavities withinthe mold. It is also important that the particles have a suitable rangeof particle sizes to enable effective tight packing of the siliconnitride particles. Tight packing of the particles minimizes theinterstitial space between particles which in turn minimizes the amountof binder required to create flow, minimizes the size of the porosity inthe sintered body and increases the sinterability—The ceramic powder maybe milled, such as in a ball mill with a solvent carrier to reduce theparticle size and achieve a desirable particle size distribution. Adesirable distribution may be non-gaussian and have a small average anda trailing larger particle size concentration. The milling and/or mixingprocesses may be performed at an elevated temperature and care may betaken to keep the elevated temperature below a temperature that wouldinitiate any chemical reactions, such as cross-linking of the binder.The solvent, ceramic powder and other components may be included duringthe milling process, such as sintering aids, dispersants, surfactantsand any other surface chemistry modifiers. A sintering aid may be anoxide ceramic, or combination of oxide ceramics, such as those selectedfrom the group comprising, Y₂O₃, Al₂O₃, MgO, BeO, silica and lanthana. Asurface modifier, such as silane or a silane containing compound, may beadded to modify the surface of the ceramic particles to promote bondingof the binder with the ceramic particles. The milling process produces amill slurry and other components may be added to the mill slurryincluding an anti-agglomerate and a binder, such as polysiloxane andpolysilazanes. The solvent may be removed to produce a moldablegreen-body composite. The solvent may be removed in a rotary evaporatoror slurry mixing kettle while the slurry is being mixed. The solvent maybe reduced down to less than 5% by weight and more preferably less than2% by weight. The green-body composite can then be further dried, suchas in a convection oven, to further reduce any solvent concentration andthen ground down to a powder. The moldable green-body composite may beground down to a size to allow uniform handling and heating duringsubsequent molding operations. The average particle size of a moldablegreen-body composite may be less than about 5 μm, less than about 2 μm,less than about 1 μm and any range between and including the particlesizes provided.

The moldable green-body composite may then be placed into a mold withpressure applied directly to the mold cavity as in compression moldingor with pressure applied to the mold cavity material via an externalsource as in injection or transfer molding. The molded part is thenthermally cured to produce a molded green body. The strength of themolded green-body part has sufficient strength to be able to handle,measure and machine (˜5-10 ksi flexure).

The molded green body may then be converted, wherein the mold is heatedto a conversion temperature for an effective conversion time to producea converted composite wherein the majority of the binder is converted tosilicon nitride. The converting step may be performed in a reactiveenvironment to allow the binder to effectively react, convert to siliconnitride and bind the ceramic particles together. The reactiveenvironment may include reactive gas mixture of hydrogen and/or ammoniawith nitrogen, for example. The hydrogen may be included in the reactivegas mixture at a low and safe concentration of no more than about 8% orno more than about 5%. Likewise, the ammonia may be included in thereactive gas mixture at a concentration of no more than about 5% or nomore than about 1%. The reactive gasses may promote conversion of thebinder to silicon nitride. In addition, the moldable green-bodycomposite may be converted under a conversion temperature profile tominimize thermal stress and the likelihood of crack and other defectformation. The green body part may be heated from about room temperatureto a conversion temperature in a ramp profile with one or moreisothermal conditions along the ramp. The strength of the converted-bodypart is sufficient to allow handling, measuring and light machininghaving about a 1 ksi flexure strength. This conversion process producesa converted composite that is then sintered to produce a molded ceramicpart.

The converted composite may be sintered in a packed powder bed at asintering temperature for a sintering time. The packed powder bed is incontact with the converted composite during the sintering step toproduce a reduced density ceramic part. The packed powder bed maycomprise, silicon nitride, boron nitride, alumina, silica; and bariumcarbonate. The sintered strength of the part is greatly increased up toabout 175 ksi flexure strength.

In an exemplary embodiment, a moldable green-body composite is producedby combining silicon nitride with a solvent, such as acetone, and asintering aid to produce a first slurry. The sintering aid may bealumina-yttria. The initial silicon nitride powder may be screenedthrough a sieve, such as a 325 mesh, which allows 44 μm or smallerparticles to pass through, prior to milling. The first slurry may beground with 0.250 in alumina grinding media. A surface modifier, such assilane, and an anti-agglomerate may be added to the mill slurry withadditional solvent, or may be pre-dissolved or mixed with a solventbefore adding. In addition, a binder, such as a polysiloxane, may beadded and may be pre-dissolved and/or mixed with a solvent beforeadding. The solvent for the binder may dissolve the binder. The bindermay be a mixture of two or more binders. In a preferred embodiment, twodifferent silicone resins are mixed to form a binder such as SILRES® REN80 and SILRES® MK Powder, available from WAKER Chemie AG. REN 80 is amethyl phenyl group containing silicone resin and MK Powder is a methylsilicone resin. In an exemplary embodiment, the binder and a solvent arecombined to produce a pre-dissolved binder having a binder concentrationof about 25 to 50%. The binder acts to hold the ceramic particlestogether after the converting step to create a converted composite. Theconverted composite is free-standing with fairly high strength and isthen sintered to from a ceramic part.

The conversion and sintering steps of the moldable green-body compositemay be conducted in a mold or in a form and a press. The green-bodycomposite may be poured or otherwise configured into a mold and thenheated to a conversion temperature for an effective amount of conversiontime to produce a converted composite. In one embodiment, the compositeis configured into a form, heated and pressed, such as in a Wabash pressunder a high load, or molding pressure. In another embodiment, themoldable green-body composite may be placed in a mold and heated withinthe mold to produce a converted composite. The time and temperatures forcuring, converting and sintering may depend on the part geometry such assize and thickness. Large parts that have thick sections may requirelonger times at temperature than small and thinner parts.

In an exemplary embodiment, the moldable green-body composite comprisesa surface modifier, such as silane, that may act to modify the surfaceof the ceramic particles for coupling with a binder, such aspolysiloxane. The surface modifier modifies the surface of the siliconnitride particles to make them more hydrophilic. In an exemplaryembodiment, a surface modifier is added to a first slurry of siliconnitride particles and a solvent, such as acetone, to produce a secondslurry.

An anti-agglomerate, such as stearic acid, is added to enhance particledispersion and to create a dispersed slurry. An anti-agglomerate may bea dispersant, surfactant or a steric hinderance agent or compound. Asteric hinderance compound may act to prevent particles from contactingeach other and agglomerating. It is important to have dispersedparticles, otherwise, agglomerated particles may lead to poor partuniformity and a weak part that is prone to cracking. A bindercomprising polysiloxane is mixed with a solvent and then added to thedispersed mill slurry to produce a green body slurry. A catalyst may beincluded to promote reaction and cross-linking of the binder. Thesolvents are substantially removed from this green body slurry toproduce an exemplary moldable silicon nitride composite. The moldablesilicon nitride composite, or moldable green-body composite, may then bepost ground to a particle size that will allow the moldable siliconnitride composite to easily flow into molds. The moldable green-bodycomposite may be ground or otherwise reduced in size to have an averageparticle size of about 10 μm or less, about 5 μm or less, and the like.

An exemplary moldable green-body composite is a solid at roomtemperature and will flow when heated. An exemplary green-body compositecomprises silicon nitride particles that have an average particle sizeand particle size distribution that enables effective flow of thecomposition when heated, such that the exemplary green-body resin can beinjection molded into cavities. An exemplary green-body composite willhave a flow length of at least about 5 in., and preferably at leastabout 10 in and even more preferably at least about 20 in, and any rangebetween and including the flow lengths provided, under a modifiedversion of ASTM D 3123, Standard Test Method for Spiral Flow ofLow-Pressure Thermosetting Molding Compounds to account for the moldingconditions including, time, temperature and pressure that give goodresults for this material. The entirety of ASTM D 3123-09 (2017),Standard Test Method for Spiral Flow of Low-Pressure ThermosettingMolding Compounds is hereby incorporated by reference herein. Thisunique flow characteristic of the green-body resin, enables the resin tobe molded into parts that can be converted and sintered into a lowdensity silicon nitride ceramic. The components of an exemplarygreen-body resin may include, but are not limited to, silicon nitride, asintering aid, a surface modifier, a binder and an anti-agglomerate. Thesilicon nitride may be included in a concentration of at least about 60%by weight, at least about 75% by weight, at least about 90%, or no morethan about 95% by weight and any range between and including theconcentrations provided. If the silicon nitride is not included in ahigh enough concentration, the resulting silicon nitride ceramic will beweak and prone to cracking during subsequent processing and if it is toohigh, the green-body resin may not flow or be injection moldable. Inaddition, the particle size of the silicon nitride is important forinjection molding flow characteristics and also for the strength of thesilicon nitride ceramic. The average particle size is preferably lessthan Sum, and more preferably less than about 2 um and even morepreferably about 1 um or less. In addition, the particle sizedistribution is important to provide tight packing of the particles toimprove flow characteristics as well as enhance the sintering process.The binder may be included in a concentration of at least about 5% byweight, at least about 10% by weight, at least about 15%, or no morethan about 25% by weight, or no more than about 20% and any rangebetween and including the concentrations provided. The sintering aid maybe included in a concentration of no more than 8% by weight, no morethan 4% by weight, or no more than about 3.5% by weight, or at leastabout 1% by weight, of the green-body resin, and any range between andincluding the values provided. The surface modifier may be included in aconcentration of about 5% or less, about 3% or less, or about 2% orless, or at least 0.3% by weight of the green-body resin and any rangebetween and including the concentrations provided. The anti-agglomeratemay be added in a concentration of about 1% or less by weight, about0.5% or less by weight of the green-body resin, or about 0.25% or moreby weight about and any range between and including the concentrationvalues provided.

In an exemplary embodiment, the dimensional changes between thegreen-body part and the sintered silicon nitride ceramic is no more than10%, preferably no more than 5%, and even more preferably no more than2%, etc. In addition, the dimensional changes may be uniform oversubstantially the entire part. These small dimensional changes enablemolds to more closely represent the final dimensions of the ceramic partand the uniformity of these dimensional changes, such as shrinkagebetween the green-body part and the ceramic part, enable oversizing ofthe mold including fastener locations, such as apertures that thenshrink to a desired dimension.

The summary of the invention is provided as a general introduction tosome of the embodiments of the invention and is not intended to belimiting. Additional example embodiments including variations andalternative configurations of the invention are provided herein.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 shows a preferred process diagram for making a moldablegreen-body composite.

FIG. 2 shows a preferred process diagram for making a molded ceramicpart from a moldable green-body composite.

FIG. 3 shows a side view of a cured green body.

FIG. 4 shows a front view of a cured green body.

FIG. 5 shows a side view of a converted part.

FIG. 6 shows a front view of a converted part.

FIG. 7 shows a side view of a molded ceramic part.

FIG. 8 shows a front view of a molded ceramic part.

FIG. 9 shows a graph of the Elevated Temperature Response for a siliconnitride ceramic made according to the present invention.

FIG. 10 shows a graph of silicon nitride particle size distribution forthe milled ceramic slurry.

Corresponding reference characters indicate corresponding partsthroughout the several views of the figures. The figures represent anillustration of some of the embodiments of the present invention and arenot to be construed as limiting the scope of the invention in anymanner. Further, the figures are not necessarily to scale, some featuresmay be exaggerated to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Also, use of “a” or “an” are employed to describeelements and components described herein. This is done merely forconvenience and to give a general sense of the scope of the invention.This description should be read to include one or at least one and thesingular also includes the plural unless it is obvious that it is meantotherwise.

Certain exemplary embodiments of the present invention are describedherein and are illustrated in the accompanying figures. The embodimentsdescribed are only for purposes of illustrating the present inventionand should not be interpreted as limiting the scope of the invention.Other embodiments of the invention, and certain modifications,combinations and improvements of the described embodiments, will occurto those skilled in the art and all such alternate embodiments,combinations, modifications, improvements are within the scope of thepresent invention.

As shown in FIG. 1, a moldable green-body resin is produced by making amill slurry that combines a ceramic powder with a solvent. Othercomponents may be added and the ceramic powder is milled to produce aceramic powder having a reduced particle size. Other components that maybe added to the mill slurry include sintering aids, surface modifier andanti-agglomerates. A binder is mixed with a solvent to pre-dissolve thebinder, which is added to the mill slurry to produce a combined slurry.The combined slurry is mixed and dried to produce a green body slurrythat is further dried and ground to produce a moldable green-bodycomposite.

As shown in FIG. 2, a green-body composite or resin is molded into amolded green-body composite and then cured by the application of heatand pressure to produce a green body part. The green body part is thenconverted by the application of heat to produce a converted part. Theconverted part is then sintered by the application of heat to make aceramic part from a moldable green-body resin. An exemplary moldablegreen-body resin is molded in a mold at a molding temperature andmolding pressure to form a molded part, the molding pressure may be atleast about 2000 psi or about 2500 psi or more or about 3000 psi ormore. An exemplary molding temperature is less than about 120° C. Anexemplary step of curing the green body is conducted at an effectivetemperature of at least 200° C. and time of at least 5 minutes or atleast 10 minutes to cross-linking the binder. The time will depend onthe size of the part. An exemplary converting temperature schedule endsin a maximum temperature of at least about 700° C. and a soak time is atleast one hour but may be at least 800° C. or about 900° C. for a longersoak time, such as at least 4 hours, or at least 6 hours, or at least 8hours. An exemplary reactive environment of the converting step includesintroduction of a reactive gas comprising nitrogen, hydrogen andammonia. An exemplary sintering temperature is no more than 1650° C.

As shown in FIGS. 3 and 4, a cured green body 10 comprises ceramicparticles 12 that are retained by a binder 14. The space betweenfastener holes is shown as 18.

As shown in FIGS. 5 and 6, a converted part 20 comprises ceramicparticles 12. The space between fastener holes is shown as 28.

As shown in FIGS. 7 and 8, a molded ceramic part 30 comprises ceramicparticles 12. The space between fastener holes is shown as 38. The spacebetween fastener holes may have less than 10% reduction from theconverted green body and the molded ceramic part.

As shown in FIG. 9, an exemplary silicon nitride ceramic made accordingto this invention from a moldable green-body composite has very gooddielectric properties. The parts tested here have a density of 75% oftheoretical.

FIG. 10, shows a graph of silicon nitride particle size distribution forthe milled ceramic slurry. The silicon nitride may have an averageparticle size of about 0.5 to 2 microns with a preferred particle sizeof about one micron with a quantity of larger particles to enhancepacking. As shown, the a quantity of larger particles trails off fromthe smaller particle sizes.

EXAMPLE

An exemplary moldable green-body composite was made according to thepresent invention with the composition as described in Table 1. Theconcentration of the silicon nitride was about 82% by weight. Theconcentration of the binder was about 15% by weight and theconcentration of surface modifier, methyltriethoxysilane. (MTES) ofabout 1% by weight. Anti-agglomeration agent, stearic acid is includedin a concentration of about 0.18% by weight. The sintering aids areinclude in a combined concentration of about 2% by weight.

TABLE 1 Mass % of Component (g) Total Si₃N₄ 300 82 Al₂O₃ 3.1 0.88 Y₂O₃4.3 1.18 Binder 54 14.77 MTES 3 0.82 Stearic acid 1.2 0.33

It will be apparent to those skilled in the art that variousmodifications, combinations and variations can be made in the presentinvention without departing from the spirit or scope of the invention.Specific embodiments, features and elements described herein may bemodified, and/or combined in any suitable manner. Thus, it is intendedthat the present invention cover the modifications, combinations andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A process for preparing a moldable green-bodycomposite comprising: a) providing silicon nitride powder; b) providinga surface modifier; c) providing a binder comprising polysiloxane orpolysilazanes, d) providing an anti-agglomerate; e) providing asintering aid; f) milling the silicon nitride powder and sintering aidwith a first solvent to create a mill slurry of silicon nitrideparticles and milling said mill slurry to produce silicon nitrideparticles having a first average particle size of no more than 5 μm; g)adding the surface modifier to the mill slurry and mixing at an elevatedtemperature to chemically modify a surface of the silicon nitrideparticles; h) adding the anti-agglomerate to the mill slurry and mixingat a first temperature to create a dispersed slurry having dispersedsilicon nitride particles; i) combining the binder and a second solventto produce a pre-dissolved binder having 25 to 50 volume percent binder;j) adding the pre-dissolved binder to the dispersed slurry and mixing ata second temperature, that is effectively low to prevent curing of thebinder, to create a green body slurry; wherein the second temperature isbelow 60° C.: k) removing the first and second solvents under a vacuumuntil the green body slurry comprises no more than about 2% by weightsolvent; thereby producing a moldable green-body composite, having abinder concentration of between 30% and 50% by volume.
 2. The processfor preparing a moldable green-body composite of claim 1, wherein thefirst average particle size is no more than 2 microns.
 3. The processfor preparing a moldable green-body composite of claim 1, furthercomprising the step of grinding the moldable green-body composite to aparticle size of no more than 5 microns.
 4. The process for preparing amoldable green-body composite of claim 1, wherein the surface modifiercomprises a silane.
 5. The process for preparing a moldable green-bodycomposite of claim 4, wherein the silane comprises tetraethyl oxysilane.6. The process for preparing a moldable green-body composite of claim 4,wherein the silane comprises methyltriethoxysilane.
 7. The process forpreparing a moldable green-body composite of claim 1, wherein theanti-agglomerate is selected from the group consisting of: a dispersant,a surfactant, a steric hinderance agent.
 8. The process for preparing amoldable green-body composite of claim 1, wherein the anti-agglomeratecomprises a steric hinderance agent comprising a stearic acid.
 9. Theprocess for preparing a moldable green-body composite of claim 1,wherein the first temperature and second temperature are less than 60°C.
 10. The process for preparing a moldable green-body composite ofclaim 1, wherein the step of milling the silicon nitride powder includesadding a sintering aid with the silicon nitride.
 11. The process forpreparing a moldable green-body composite of claim 1, wherein thesintering aid is added in an amount of no more than 10% by weight of thesilicon nitride.
 12. The process for preparing a moldable green-bodycomposite of claim 1, wherein the sintering aid is added in an amount ofno more than 5% by weight of the silicon nitride.
 13. The process forpreparing a moldable green-body composite of claim 12, wherein thesintering aid is an oxide ceramic.
 14. The process for preparing amoldable green-body composite of claim 12, wherein the sintering aid isselected from the group consisting of: yttria-alumina, Al₂O₃, MgO, BeO,silica and lanthana.
 15. The process for preparing a moldable green-bodycomposite of claim 1, wherein the first solvent dissolves the binder.